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Zzzzzzzz
We have to do it every night but nobody knows why
AT THE END OF A CENTURY when the mysteries of life have fallen one after
another to the advance of molecular biology, it is reassuring to find that
we still have one big puzzle for the next century's scientists to get their
teeth into. Even more delightful, it's a puzzle that can be expressed in
just four words: "Why do we sleep?"
Nobody knows the answer, despite fifty years of intensive research. Everyone
knows, of course, that we fall asleep because we are tired and somehow sleep
restores our sense of wellbeing. But what exactly does sleep revive and why?
And why can't we do without it?
A popular view is that we sort out the day's experiences and reinforce
memories during sleep, perhaps through a subtle transfer of information
between different parts of the brain (see p 26). But the trouble with such
theories is that they are both too weak and too strong. On the one hand,
they don't explain the absolute need we have for sleep. If totally deprived
of sleep, animals don't just become a little confused about remembering
their way through a maze, they actually die--and almost as quickly as if
deprived of food (see New Scientist supplement, 26 April 1997). On the other
hand, the theories don't really explain why we can handle a few nights
without sleep and not suffer serious mental impairment. If you don't count a
tendency to bite everybody's head off, that is.
Where can we look for an answer? The usual place to start when tackling big
"what for" questions is in our own evolutionary past. Much of our physiology
and behaviour can be understood by studying other animals, but when we turn
to sleep the plot merely thickens.
It is essential, of course, to distinguish between the two major phases of
sleep--slow-wave sleep (SWS) and rapid eye movement (REM) or dreaming sleep.
Dreaming is restricted to the higher animals-- mammals, birds and some
reptiles--and thus seems likely to have a cognitive function. But even that
rule has its strange exceptions. Although we might forgive the echidna, one
of the most primitive mammals, for not indulging in REM sleep, why is it
that dolphins and whales also seem not to dream?
How long animals sleep is also very variable. Bats sleep for up to 20 hours
a day. Both the giant anteater and the giant sloth also put in 18 hours, but
the anteater dreams for around 7 hours while the sloth makes do with 70
minutes. Is chasing ants really such a cognitive strain? Lions can sleep for
days on end. But a shrew sleeps for only a few hours.
Generally speaking, animals that are secure from predators sleep a lot,
while those whose lives are at risk sleep little. That could suggest that
sleep is designed simply to conserve energy. Evolution follows the philos-
ophy of the couch potato--if you are well fed and secure, just take it easy.
But even here, there are exceptions. Elephants have few predators but sleep
little. The Indus dolphin lives in a dangerous environment but manages to
clock up 7 hours a day.
Perhaps all we can really be certain of is that although the amount and type
of sleep is in some way related to lifestyle, no higher animal can survive
without at least some sleep. Which suggests that the roots of sleep run very
deep. Indeed, all animals go though cycles of activity and rest, even the
humble amoeba. Sleep has always seemed to be something more complicated. But
perhaps, after all, it is not--a view recently expressed Giulio Tononi of
the The Neurosciences Institute in San Diego and one of the leaders in the
field, in the question: "Can single neurons sleep?" In other words, we still
don't really even know whether sleep is a local or a global phenomenon of
nervous systems.
If global, then we can expect neurobiologists to crack the problem of what
sleep is for. If local, then molecular biologists will find the answer
inside single cells. And if sleep is just nature's way of stopping us
wasting precious energy then it will be behavioural ecologists who strike it
lucky with the right theory.
Whatever the outcome, it's good to know that we have a deep mystery left.